1. Field of the Invention
The present invention concerns a control device for supplying desired liquid volumes by means of a metering pump even if, owing to external reasons, the flow rate of the pump would be variable.
2. Description of the Prior Art
In many industrial processes connected with automatic processing or packaging, such as automatic filling of containers or cans, and as in many manual or automatic machines for distributing beverages and specifically warm beverages such as coffee percolators and automatic machines for preparing and distributing the warm beverages, there is the problem of supplying desired liquid volumes which are not always equal to each other, but which must be selected every time in a simple and immediate way.
A very simple and primitive but rather precise, method for supplying desired liquid volumes, would be to fill in with the liquid a number of containers having a known capacity and then to draw the liquid from the containers in order to have them completely emptied. This method has the drawback of requiring a certain number of container, having calibrated volume which, when in the case of a wide metering range may be untollerably high, adding encumbrance drawbacks and reducing the metering accuracy because of long connecting ducts introducing hardly evaluable and controllable supplementary capacities. This drawback can be partially overcome by using, just one graduated container (such as a graduated glass) allowing as many different capacities as is the number of graduations on the container. This solution is however less precise in the metering because of unavoidable inaccuracies in calibrating the graduated container and in detecting the liquid level in the container. A more practical method, fully independent from calibrated containers, would be that of using a positive displacement pump such as, for example a piston pump, having a constant stroke length and powered by a motor through a crank and link assembly, calibrated so as to stop after a preset number of strokes, estimated for example by counting, the number of revolutions made by the motor. Such a method can produce rather accurate meterings, but the use of an expensive device such as the positive displacement pump and the problem of carefully stopping the pump at the expected time, adds rather high costs and can lead to such an expensive device which can not be installed in all apparatus, because the expense of such a solution is not always bearable.
A third method is to use piston pumps of the vibrating type comprising an electromagnet provided with a movable core rigidly connected to a piston movable within a cylinder whereby for any displacement of the movable core a like displacement of the piston corresponds thereto. These pumps are also termed "electromagnetic pumps".
In such a kind of pump, in order to cause an alternative displacement of the piston, the coil winding of the electromagnet can be supplied with an alternating current or, better, with an unidirectional pulsating current, such as that obtained by rectifying just a halfwave of a usual a.c. electric current. It is to be pointed out that when a current pulse is applied to the electromagnet of the above mentioned vibrating pump, the movable core is attracted within the electromagnet, causing a suction stroke of the piston and compressing a spring which is thus loaded during the suction stroke and is released when the current through said electromagnet is switched off giving a compression stroke. Thus, if the pump operates under unchanging conditions, i.e. having to overcome always the same resistance due to the backpressure, frictions, dilatations, wears, etc. . , its stroke remains constant and every pump stroke provides a constant liquid flow rate.
Under such ideal conditions, being that the liquid flow is constant for every pump stroke, in order to obtain a desired liquid volume, it is sufficient to calculate the corresponding number of pump strokes and thus the number of current pulses applied to the electromagnet from the desired volume divided by the volume delivered by a single pump stroke.
As a consequence, in order to have a desired liquid volume delivered it is sufficient to apply a corresponding number of current pulses to the pump electromagnet. In order to count the number of current pulses applied to the pump electromagnet, it is sufficient to derivate from the voltage across the electromagnet coil, pulse signals to be counted by a digital counter starting with the first pulse received when a voltage begins to be applied to the electromagnet and, stopping after having counted a pulse number corresponding to the desired volume, care being also taken to stop the voltage application to the electromagnet, whereby the pump does affect just the desired stroke number to obtain the desired liquid volume. This system, by means of which pulses are generated from the network alternating current at the fixed frequency thereof, is basically objectionable owing to the resistances, to be overcome by the pump, which affect its stroke and thus the volume delivered by each single pump stroke. Specifically if the backpressure to be overcome by the pump varies, the flow rate thereof does also vary, and decreases when the backpressure increases. The main related drawback is that, the pump stroke number being the same, different liquid volumes are delivered depending on the backpressure to be overcome and thus not only the liquid volume delivered by the given pump stroke number can vary depending on the application, but it can also vary for the same application if the pump must inject a liquid in a pressurized vessel overcoming an increasing backpressure or anyhow variable resistances due to variable flow rate adjustments must be overcome, so that even for the same application the simple counting of the current pulses applied to an electromagnet cannot any longer reliable.
As above mentioned the pump stroke variation as caused from the resistances to be overcome, originates from the fact that the vibrating pump comprises a piston which is moved by a movable core electromagnet for withdrawing the core when the electromagnet is energized by a current, providing a suction stroke of the piston, and releasing it under the action of a spring loaded during the withdrawing motion, when the current through the electromagnet is switched off; if the pumps are supplied with current pulses having fixed a repetition frequency, it may occur that, in case rather high resistances must be overcome, as for backpressure increases, the compression stroke of the pump is not ended before the arrival of next current pulse and, as a suction stroke begins before the end of the preceeding compression stroke, the length of the latter compression stroke is reduced and the volume delivered by the same number of pump strokes is correspondingly reduced.
In order to obviate such a serious problem, it has been suggested to feed the pump with current pulses having a variable frequency, the beginning of which is controlled by a specific position of the piston within the cylinder so that a suction stroke always begins when the preceeding compression stroke is completely ended, thus ensuring a compression stroke having constant length and thus a constant flow rate of said pump so that an equal pump stroke number corresponds to an equal volume of delivered liquid.
A pump according to the above mentioned feature is described in the Italian utility model application No. 21796 B/84 having the same inventor as the present application and filed on May 15, 1984. In the application there is described a vibrating pump provided with a piston position detector for detecting the piston position which is energized by the piston taking a determined position within the pump cylinder or body, allowing the application of a current pulse to the pump electromagnet only when the piston reaches the position, the piston position detector controlling a driving circuit for the electromagnet and sending also a pulse signal to a digital counter whose counting is increased by one unit every time the piston reaches the position in the pump casing. The counter emits a stopping pulse to the driving circuit, which had been started by a starting control, when a desired counting corresponding to the volume to be delivered by the pump is reached.
The pump of the above mentioned utility model application is rather satisfactory as a metering pump in variable flow rate condition, leading to rather precise metering, specifically in the automatic field of the beverage automatic distributing machines and particularly in the field of the coffee preparing machines. However, it has the main drawback of allowing just one desired metered liquid volume to be delivered, unless a plurality of counters is adopted emitting a stopping signal for the electromagnet at the end of different countings corresponding to different metered volumes.